The
iostat
command is used for monitoring system input/output device
loading by observing the time the devices are active in relation
to their average transfer rates. The
iostat
command generates reports
that can be used to change system configuration to better balance
the input/output load between physical disks.
The first report generated by the
iostat
command provides statistics
concerning the time since the system was booted. Each subsequent report
covers the time since the previous report. All statistics are reported
each time the iostat command is run. The report consists of a
CPU header row followed by a row of
CPU statistics. On
multiprocessor systems, CPU statistics are calculated system-wide
as averages among all processors. A device header row is displayed
followed by a line of statistics for each device that is configured.
The
interval
parameter specifies the amount of time in seconds between
each report. The first report contains statistics for the time since
system startup (boot). Each subsequent report contains statistics
collected during the interval since the previous report. The
count
parameter can be specified in conjunction with the
interval
parameter. If the
count
parameter is specified, the value of
count
determines the number of reports generated at
interval
seconds apart. If the
interval
parameter is specified without the
count
parameter, the
iostat
command generates reports continuously.
REPORTS
The
iostat
command generates two types of reports, the CPU
Utilization report and the Device Utilization report.
CPU Utilization Report
The first report generated by the
iostat
command is the CPU
Utilization Report. For multiprocessor systems, the CPU values are
global averages among all processors.
The report has the following format:
%user
Show the percentage of CPU utilization that occurred while
executing at the user level (application).
%nice
Show the percentage of CPU utilization that occurred while
executing at the user level with nice priority.
%system
Show the percentage of CPU utilization that occurred while
executing at the system level (kernel).
%iowait
Show the percentage of time that the CPU or CPUs were idle during which
the system had an outstanding disk I/O request.
%steal
Show the percentage of time spent in involuntary wait by the virtual CPU
or CPUs while the hypervisor was servicing another virtual processor.
%idle
Show the percentage of time that the CPU or CPUs were idle and the system
did not have an outstanding disk I/O request.
Device Utilization Report
The second report generated by the
iostat
command is the Device Utilization
Report. The device report provides statistics on a per physical device
or partition basis. Block devices for which statistics are to be displayed
may be entered on the command line. Partitions may also be entered on the
command line providing that option -x is not used.
If no device nor partition
is entered, then statistics are displayed
for every device used by the system, and
providing that the kernel maintains statistics for it.
If the
ALL
keyword is given on the command line, then statistics are
displayed for every device defined by the system, including those
that have never been used.
The report may show the following fields,
depending on the flags used:
Device:
This column gives the device (or partition) name, which is displayed as
hdiskn
with 2.2 kernels, for the nth device. It is displayed as
devm-n
with 2.4 kernels, where
m
is the major number of the device, and
n
a distinctive number.
With newer kernels, the device name as listed in the /dev directory
is displayed.
tps
Indicate the number of transfers per second that were issued
to the device. A transfer is an I/O request to the
device. Multiple logical requests can be combined into a single I/O
request to the device. A transfer is of indeterminate size.
Blk_read/s
Indicate the amount of data read from the device expressed in a number of
blocks per second. Blocks are equivalent to sectors with 2.4 kernels and newer
and therefore have a size of 512 bytes. With older kernels, a block is of
indeterminate size.
Blk_wrtn/s
Indicate the amount of data written to the device expressed in a number of
blocks per second.
Blk_read
The total number of blocks read.
Blk_wrtn
The total number of blocks written.
kB_read/s
Indicate the amount of data read from the device expressed in kilobytes
per second.
kB_wrtn/s
Indicate the amount of data written to the device expressed in kilobytes
per second.
kB_read
The total number of kilobytes read.
kB_wrtn
The total number of kilobytes written.
MB_read/s
Indicate the amount of data read from the device expressed in megabytes
per second.
MB_wrtn/s
Indicate the amount of data written to the device expressed in megabytes
per second.
MB_read
The total number of megabytes read.
MB_wrtn
The total number of megabytes written.
rrqm/s
The number of read requests merged per second that were queued to the device.
wrqm/s
The number of write requests merged per second that were queued to the device.
r/s
The number of read requests that were issued to the device per second.
w/s
The number of write requests that were issued to the device per second.
rsec/s
The number of sectors read from the device per second.
wsec/s
The number of sectors written to the device per second.
rkB/s
The number of kilobytes read from the device per second.
wkB/s
The number of kilobytes written to the device per second.
rMB/s
The number of megabytes read from the device per second.
wMB/s
The number of megabytes written to the device per second.
avgrq-sz
The average size (in sectors) of the requests that were issued to the device.
avgqu-sz
The average queue length of the requests that were issued to the device.
await
The average time (in milliseconds) for I/O requests issued to the device
to be served. This includes the time spent by the requests in queue and
the time spent servicing them.
svctm
The average service time (in milliseconds) for I/O requests that were issued
to the device.
%util
Percentage of CPU time during which I/O requests were issued to the device
(bandwidth utilization for the device). Device saturation occurs when this
value is close to 100%.
OPTIONS
-c
The -c option is exclusive of the -d option and displays only the
CPU usage report.
-d
The -d option is exclusive of the -c option and displays only the
device utilization report.
-k
Display statistics in kilobytes per second instead of blocks per second.
Data displayed are valid only with kernels 2.4 and newer.
-m
Display statistics in megabytes per second instead of blocks or
kilobytes per second.
Data displayed are valid only with kernels 2.4 and newer.
-n
Displays the NFS-directory statistic.
Data displayed are valid only with kernels 2.6.17 and newer.
This option is exclusive ot the -x option.
-p [ { device | ALL } ]
The -p option is exclusive of the -x option and displays statistics for
block devices and all their partitions that are used by the system.
If a device name is entered on the command line, then statistics for it
and all its partitions are displayed. Last, the
ALL
keyword indicates that statistics have to be displayed for all the block
devices and partitions defined by the system, including those that have
never been used.
Note that this option works only with post 2.5 kernels.
-t
Print the time for each report displayed.
-V
Print version number then exit.
-x
Display extended statistics.
This option is exclusive of the -p and -n, and works with
post 2.5 kernels since it needs /proc/diskstats file or a mounted sysfs to
get the statistics. This option may also work with older kernels (e.g. 2.4)
only if extended statistics are available in /proc/partitions (the kernel
needs to be patched for that).
ENVIRONMENT
The
iostat
command takes into account the following environment variable:
S_TIME_FORMAT
If this variable exists and its value is
ISO
then the current locale will be ignored when printing the date in the report
header. The
iostat
command will use the ISO 8601 format (YYYY-MM-DD) instead.
EXAMPLES
iostat
Display a single history since boot report for all CPU and Devices.
iostat -d 2
Display a continuous device report at two second intervals.
iostat -d 2 6
Display six reports at two second intervals for all devices.
iostat -x hda hdb 2 6
Display six reports of extended statistics at two second intervals for devices
hda and hdb.
iostat -p sda 2 6
Display six reports at two second intervals for device sda and all its
partitions (sda1, etc.)
BUGS
/proc
filesystem must be mounted for
iostat
to work.
Extended statistics are available only with
post 2.5 kernels.
FILES
/proc/stat
contains system statistics.
/proc/partitions
contains disk statistics (for pre 2.5 kernels that have been patched).
/proc/diskstats
contains disks statistics (for post 2.5 kernels).
/sys
contains statistics for block devices (post 2.5 kernels).
